Acceleration sensor and method of manufacturing acceleration sensor

ABSTRACT

An acceleration sensor includes a semiconductor substrate, a sensing element formed on the semiconductor substrate, a bonding frame made of polysilicon which is formed on the semiconductor substrate and surrounds the sensing element, and a glass cap which is bonded to a top surface of the bonding frame made of polysilicon to cover the sensing element above the sensing element while being spaced by a predetermined distance from the sensing element. The bonding frame made of polysilicon is not doped with any impurity.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/219,715,filed Sep. 7, 2005 now U.S. Pat. No. 7,071,549, which is a divisional ofSer. No. 10/859,218, filed Jun. 3, 2004, now U.S. Pat. No. 6,953,993,which claims priority of Japanese Application No. 2003-411319, filedDec. 10, 2003, the contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acceleration sensor, and moreparticularly to an acceleration sensor including a cap which covers asensing element while being spaced by a predetermined distance from thesensing element.

2. Description of the Background Art

In recent days, an air bag system has been employed in most ofautomobiles on the market. Typically, an air bag system includes anacceleration sensor for detecting impact.

In order to make an acceleration sensor adaptable to incorporation invarious types of automobiles, there has conventionally been efforts toreduce a size of an acceleration sensor, as well as its associatedcosts. For example, concerning a package for covering a semiconductorsubstrate which forms an acceleration detector and a signal processor ofan acceleration sensor, metal which had been most widely used as amaterial for the package has been superseded by resin.

In the meantime, turning to a structure of an acceleration sensor, asensing element including a mass body which is a movable portion fordetecting an acceleration and the like is formed on a surface of asemiconductor substrate in an acceleration detector. Further, a glasscap is bonded to the surface of the semiconductor substrate in order tokeep an admissible motion space of the mass body and prevent entry ofdust, water or the like into the admissible motion space. By provisionof the glass cap, the admissible motion space of the mass body iscompletely enclosed.

More specifically, a bonding frame which surrounds the sensing elementformed on the semiconductor substrate in plan view is formed on thesemiconductor substrate. It is noted that polysilicon doped withimpurities which is used as a material for the sensing element is alsoused as a material for the bonding frame. The bonding frame is incontact with an end portion of the glass cap. With the bonding frame andthe end portion of the glass cap being kept in contact with each other,the glass cap and the semiconductor substrate are bonded to each otherby anodic bonding for purposes of improving heremeticity or the like(please refer to National Publication of Translation No. 2002-500961 andJapanese Patent Application Laid-Open No. 9-292409, for example).

However, since the bonding frame is formed of polysilicon doped withimpurities, the above-described structure has suffered from problemsassociated with anodic bonding, as follows.

Upon application of a voltage in anodic bonding, the impuritiescontained in the bonding frame are precipitated out in a portion of thebonding frame near a bonding interface. As anodic bonding to bond theglass cap and the bonding frame is achieved by uniting together glass ofthe glass cap and polysilicon of the bonding frame, the precipitatedimpurities existing between the glass of the glass cap and thepolysilicon of the bonding frame would reduce a bonding strength betweenthe glass cap and the bonding frame.

That is, to allow the impurities to be precipitated out near the bondinginterface would cause reduction of the bonding strength between theglass cap and the bonding frame.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an accelerationsensor which provides for improvement of a bonding strength between abonding frame and a cap, and a method of manufacturing such anacceleration sensor.

According to a first aspect of the present invention, an accelerationsensor includes a substrate, a sensing element, a polysilicon bondingframe and a cap. The sensing element is formed on the substrate. Thepolysilicon bonding frame is formed on the substrate, and surrounds thesensing element in plan view. The cap includes an end face bonded to atop surface of the polysilicon bonding frame. Also, the cap covers thesensing element above the sensing element while being spaced by apredetermined distance from the sensing element. The polysilicon bondingframe is not doped with any impurity.

Even if anodic bonding between the polysilicon bonding frame and thecap, is carried out, no impurity is precipitated out in a portion of thepolysilicon bonding frame near a bonding interface between the cap andthe polysilicon bonding frame. Thus, precipitation of an impurity nearthe bonding interface which has disadvantageously caused reduction of abonding strength of a bond resulted from anodic bonding in accordancewith the conventional practices can be prevented. Hence, it is possibleto bond the polysilicon bonding frame and the cap more strongly.

According to a second aspect of the present invention, an accelerationsensor includes a substrate, a sensing element, a bonding frame and acap. The sensing element is formed on the substrate. The bonding frameis formed on the substrate and surrounds the sensing element in planview. The cap includes an end face bonded to a top surface of thebonding frame. Also, the cap covers the sensing element above thesensing element while being spaced by a predetermined distance from thesensing element. The bonding frame includes a first polysilicon layer,an insulating film and a second polysilicon layer. The first polysiliconlayer is doped with an impurity. The insulating film is formed on thefirst polysilicon layer. The second polysilicon layer is formed on theinsulating film, and not doped with any impurity. The cap is bonded tothe second polysilicon layer.

The acceleration sensor according to the second aspect of the presentinvention can produce the same effects as produced by the accelerationsensor according to the first aspect of the present invention, andfurther, allows a height of the bonding frame to be easily controlled.Moreover, even if anodic bonding between the bonding frame and the capis carried out, it is possible to prevent the impurity contained in thefirst polysilicon layer from diffusing into the second polysilicon layerbecause of the provision of the insulating film between the firstpolysilicon layer and the second polysilicon layer.

According to a third aspect of the present invention, an accelerationsensor includes a substrate, a sensing element, a polysilicon bondingframe, a cap, a metal film, a first pad, a second pad and a third pad.The sensing element is formed on the substrate. The polysilicon bondingframe is formed on the substrate and surrounds the sensing element inplan view. The cap includes an end face bonded to a top surface of thepolysilicon bonding frame. Also, the cap covers the sensing elementabove the sensing element while being spaced by a predetermined distancefrom the sensing element. The metal film is formed on a surface of thecap which faces the sensing element. Also, a portion of the metal filmis bonded to the polysilicon bonding frame. The first pad is formed onthe substrate and is electrically connected to the sensing element via afirst interconnect line. The second pad is electrically connected to thesubstrate. The third pad is formed on the substrate and is electricallyconnected to the polysilicon bonding frame via a second interconnectline.

With all of the first pad, the second pad and the third pad beingelectrically connected to one another, anodic bonding between thepolysilicon bonding frame and the cap is carried out. In this manner,even with a voltage being applied in anodic bonding, the substrate, thepolysilicon bonding frame, the sensing element, the metal film and thelike are held at the same potential by virtue of electrical connectionamong the first, second and third pads. Accordingly, even if anodicbonding is carried out, no electrostatic force is caused between thesensing element and the other elements, so that it is possible toprevent the sensing element from being attracted to, and moving toward,the other elements.

According to a fourth aspect of the present invention, a method ofmanufacturing an acceleration sensor includes the steps (a), (b) and(c). The acceleration sensor includes a substrate, a sensing element, apolysilicon bonding frame, a cap, a first metal film, a first pad, asecond pad and a third pad. The sensing element is formed on thesubstrate. The polysilicon bonding frame is formed on the substrate andsurrounds the sensing element in plan view. The cap includes an end facebonded to a top surface of the polysilicon bonding frame. Also, the capcovers the sensing element above the sensing element while being spacedby a predetermined distance from the sensing element. The first metalfilm is formed on a surface of the cap facing the sensing element. Also,a portion of the first metal film is bonded to the polysilicon bondingframe. The first pad is formed on the substrate and is electricallyconnected to the sensing element via a first interconnect line. Thesecond pad is electrically connected to the substrate. The third pad isformed on the substrate and is electrically connected to the polysiliconbonding frame via a second interconnect line. The step (a) is to form asecond metal film for electrically connecting the first pad, the secondpad and the third pad. The step (b) is to carry out anodic bondingbetween the cap and the polysilicon bonding frame by applying a voltagebetween the substrate and the cap after the step (a). The step (c) is toremove the second metal film after the step (b).

Even if anodic bonding between the polysilicon bonding frame and the capis carried out, respective elements (the sensing element, the cap andthe like) are held at the same potential. Accordingly, it is possible toprevent the sensing element from moving toward the cap, for example,during anodic bonding.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an overall structure of an acceleration sensor.

FIG. 2 is a plan view of an acceleration detecting chip according to afirst preferred embodiment.

FIG. 3 is a sectional view of the acceleration detecting chip accordingto the first preferred embodiment.

FIG. 4 is an enlarged sectional view of a bonding frame and itsperipheral region according to the first preferred embodiment.

FIG. 5 is a sectional view for illustrating a case in which theacceleration detecting chip according to the first preferred embodimentincludes a flat cap.

FIG. 6 is a sectional view for illustrating a state in which a metalfilm is formed in a cap of an acceleration detecting chip according to asecond preferred embodiment.

FIG. 7 is a sectional view for illustrating a state in which a metalfilm with a lengthened portion is formed in the cap of the accelerationdetecting chip according to the second preferred embodiment.

FIG. 8 is a plan view of a structure of the acceleration detecting chipaccording to the second preferred embodiment.

FIG. 9 illustrates a state in which respective pads are electricallyconnected to one another in the acceleration detecting chip according tothe second preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, the present invention will be described in more detail by makingreference to the accompanying drawings which illustrate preferredembodiments of the present invention.

First Preferred Embodiment

FIG. 1 is a top view illustrating an overall structure of anacceleration sensor according to a first preferred embodiment. Asillustrated in FIG. 1, the acceleration sensor according to the firstpreferred embodiment includes a signal processing chip CP1, anacceleration detecting chip CP2 functioning as an acceleration detector,a lead LD, and a die pad DP.

Both the signal processing chip CP1 and the acceleration detecting chipCP2 are mounted on the die pad DP. A pad PD1 of the accelerationdetecting chip CP2 and a pad PD2 of the signal processing chip CP1 areconnected to each other via a wire WR1, while a pad PD3 of the signalprocessing chip CP1 and the lead LD are connected to each other via awire WR2. The foregoing elements included in the acceleration sensoraccording to the first preferred embodiment which are arranged asdescribed above are covered with a package PK made of resin.

In this regard, a portion of the lead LD is not covered with, andprotrudes from, the package PK made of resin, to function as a terminalconnected to an external component. It is noted that FIG. 1 illustratesa structure inside the package PK made of resin when it is seen throughthe package PK which is denoted by hidden lines (broken lines).

FIG. 2 is a top view of the acceleration detecting chip CP2 and FIG. 3is a sectional view of the acceleration detecting chip CP2. Thesectional view of FIG. 3 is taken along a line III-III of FIG. 2.

As illustrated in FIGS. 1, 2 and 3, a cap CA made of glass or the likeis bonded to a surface of a semiconductor substrate SB of theacceleration detecting chip CP2. It is noted that FIG. 2 illustrates astructure inside the cap CA when it is seen through the cap CA which isdenoted by hidden lines (broken lines).

Referring to FIG. 2, pads PD1 a, PD1 b, PD1 c and PD1 d which form thepad PD1 are formed in a portion of the semiconductor substrate SBincluding the surface of the semiconductor substrate SB (“surfaceportion”). Also, interconnect lines LNa, LNb, LNc and LNd which areconnected to the pads PD1 a, PD1 b, PD1 c and PD1 d, respectively, areprovided in the surface portion of the semiconductor substrate SB, asillustrated in FIGS. 2 and 3. Further, a shield electrode SE connectedto the interconnect line LNc is formed in the surface portion of thesemiconductor substrate SB.

Moreover, a sensing element made of polysilicon in the form of a thinfilm is formed on the semiconductor substrate SB. The sensing element isdoped with impurities such as phosphorus, in order to make the sensingelement electrically conductive. It is additionally noted that gallium,boron, arsenic and the like can be employed as impurities for doping, inplace of phosphorus.

The sensing element includes a mass body MS for detecting anacceleration, a fixed electrode FE1 connected to the interconnect lineLNd, a fixed electrode FE2 connected to the interconnect line LNa, asupport SP which supports the mass body MS and is connected to theinterconnect line LNb, and the like. The mass body MS is jointed to thesupport SP via a beam BM, and thus is held in the air as illustrated inFIG. 3.

The sensing element on the semiconductor substrate SB is manufactured bysemiconductor manufacturing processes. More specifically, respectiveelements illustrated in FIGS. 1, 2 and 3 are manufactured by utilizingtechniques such as photolithography, etching, ion implantation ofimpurities, and the like. For example, the mass body MS, the fixedelectrodes FE1 and FE2, the support SP, the beam BM and the like aremanufactured so as to exhibit the configuration as illustrated in thefigures by epitaxially growing silicon on the semiconductor substrate SBand patterning the epitaxially grown silicon with the use ofphotolithography and etching.

A movable electrode ME shaped like a comb is provided in each ofopposite wing portions of the mass body MS. The fixed electrodes FE1 andFE2 are arranged so as to face the movable electrode ME. Given sucharrangement, when accelerated, the beam BM is bent to shift the positionof the mass body MS, so that a distance between the movable electrode MEand each of the fixed electrodes FE1 and FE2 is changed. The change ofthe distance between the movable electrode ME and each of the fixedelectrodes FE1 and FE2 is followed by change of an electrostaticcapacitance existing between the movable electrode ME and each of thefixed electrodes FE1 and FE2. Accordingly, by monitoring change of anelectrostatic capacitance between the movable electrode ME and each ofthe fixed electrodes FE1 and FE2, it is possible to detect anacceleration.

That is, the mass body MS functions as a movable portion for detectingan acceleration, and the fixed electrodes FE1 and FE2 and the movableelectrode ME of the mass body MS function as an acceleration detector.

The cap CA is bonded to a bonding frame FD formed on the semiconductorsubstrate SB, to keep and completely enclose an admissible motion spaceof the mass body MS.

More specifically, the bonding frame FD is formed on the semiconductorsubstrate SB so as to surround the sensing element in plan view, asillustrated in FIGS. 2 and 3.

It is noted that a portion of the bonding frame FD (a portion denoted bya reference numeral “FD1” in FIG. 4) is manufactured in the same stepthat is performed for manufacturing the sensing element. Accordingly,the portion of the bonding frame FD (the portion denoted by thereference numeral “FD1” in FIG. 4) is formed of polysilicon doped withimpurities such as phosphorus, as is the case with the sensing element.

A top surface of the bonding frame FD and an end face of the cap CA arebonded to each other so that a top surface of the sensing element iscovered with the cap CA while being spaced by a predetermined distancefrom the cap CA. The sensing element is totally enclosed by bonding thecap CA and the bonding frame FD to each other.

The cap CA is made of glass, and bonded to the bonding frame FD byanodic bonding. It is noted that anodic bonding is a method of bondingglass containing an alkaline metal ion such as sodium or lithium and ametal (or a semiconductor) as an anode to each other, and isaccomplished by applying a several hundred voltage across the glass andthe metal at a temperature of approximately 400° C. at which thermaldiffusion of the alkaline metal ion can take place. In general, anodicbonding is carried out for a period of time in a range of several tensof minutes to several hours.

Anodic bonding may be carried out in vacuum. Alternatively, anodicbonding may be carried out under conditions where a predeterminedpressure is maintained by using an inert gas, in which case a pressureinside the cap CA which is provided after the cap CA is bonded to thebonding frame FD can be controlled to be equal to the predeterminedpressure.

Further, a side face of the cap CA includes a trapezoidal sloping facewith a width which is the smallest at a junction with the top surface ofthe cap CA and increases as a distance from a bonding interface betweenthe cap CA and the bonding frame FD decreases. Reasons for suchconfiguration of the side face of the cap CA will be set out as follows.

In carrying out a molding process for forming the package made of resinwhich covers the acceleration sensor, a force caused due to thermalshrinkage or the like of the package made of resin is applied to theside face of the cap CA. The force applied to the side face of the capCA is transmitted to the semiconductor substrate SB via the bondingframe FD. A force acting on the semiconductor substrate SB in adirection perpendicular to the semiconductor substrate SB would not sosignificantly affect the sensing element even if the force istransmitted to the semiconductor substrate SB. However, if a forceacting on the semiconductor substrate SB in a direction horizontal tothe semiconductor substrate SB is transmitted to the semiconductorsubstrate SB, the force in the horizontal direction correspondinglyaffects the sensing element, to cause degradation in accuracy of thesensing element.

In view of this, the side face of the cap CA is configured to be slopingand have the above-mentioned trapezoidal shape. Because of thisconfiguration, a force applied to the side face of the cap CA during themolding process for formation of the package made of resin can bedispersed in the direction perpendicular to the semiconductor substrateSB.

Accordingly, a force acting on the semiconductor substrate SB in thedirection horizontal to the semiconductor substrate SB can be reduced ascompared to a configuration in which the side face of the cap CA standsupright relative to a main surface of the semiconductor substrate SB. Asa result, the accuracy of the sensing element can be improved.

It is additionally noted that to form the above-noted trapezoidalsloping face in at least a portion of the side face of the cap CA couldsuffice for producing the above effects, though to do so may lessen theabove effects as compared to a case in which the entire side face of thecap CA is made sloping and trapezoidal with a width which increases as adistance from the bonding interface decreases.

Further, a recess portion CAa is formed in the cap CA in order to allowthe cap CA to cover the top surface of the sensing element while beingspaced by a predetermined distance from the top surface of the sensingelement. The formation of the recess portion CAa makes it possible toprevent the sensing element from coming into contact with the cap CA.The recess portion CAa is formed by carrying out etching or sandblastingon a surface of the cap CA which faces the sensing element.

A depth of the recess portion CAa is determined depending on a thicknessof the sensing element. For example, by making the depth of the recessportion CAa approximately equal to the thickness of the sensing element,it is possible to limit movement of the sensing element toward the capCA. On the other hand, to make a distance between the sensing elementand the cap CA relatively small would produce another effects.Specifically, even if the sensing element moves toward the cap CA andcollides against the cap CA, damages to the sensing element which mightbe caused due to the collision can be minimized because collision energydelivered to the sensing element is small by virtue of the relativelysmall distance between the sensing element and the cap CA.

Next, a structure of the bonding frame FD will be described in moredetail by making reference to FIG. 4. FIG. 4 is an enlarged view of acircled portion in FIG. 3.

As illustrated in FIG. 4, the bonding frame FD includes a layeredstructure in which a doped polysilicon layer FD1 which is doped withimpurities, an insulating film FD2 such as an oxide film or a nitridefilm, and an undoped polysilicon layer FD3 which is not doped with anyimpurity are deposited sequentially in the order of occurrence in thisexplanation.

To achieve anodic bonding between the bonding frame FD with theforegoing layered structure and the cap CA requires application of avoltage which is approximately twice a voltage applied in anodic bondingbetween polysilicon doped with impurities and a glass cap, because ofthe inclusion of the undoped polysilicon layer FD3.

More specifically, a voltage applied in anodic bonding betweenpolysilicon doped with impurities and a glass cap is approximately 200V, and a voltage applied in anodic bonding between the undopedpolysilicon layer FD3 and the cap CA is approximately 400 V. It is notedthat conditions regarding beat treatment in anodic bonding are the samewhether polysilicon subjected to anodic bonding is doped with impuritiesor not.

In the foregoing layered structure of the bonding frame FD, theinsulating film FD2 functions to suppress diffusion of the impuritiescontained in the doped polysilicon layer FD1 which are likely to diffuseduring anodic bonding between the bonding frame FD and the cap CA. As aresult, it is possible to prevent diffusion of the impurities into theundoped polysilicon layer FD3.

Thus, even if anodic bonding is carried out, no impurity is precipitatedout in a portion of the undoped polysilicon layer FD3 near the bondinginterface, to thereby overcome the problems caused in the structure setforth in the “Background” section of the present specification (that is,the problems associated with anodic bonding between a glass cap andpolysilicon doped with impurities). In particular, reduction of abonding strength can be prevented, to thereby provide for improvement ina bonding strength between the bonding frame FD and the cap CA.

It is noted that though the above description has been made about a casewhere the bonding frame FD includes a layered structure, the bondingframe FD may include only the undoped polysilicon layer FD3,alternatively. In this alternative, there is a need of covering thebonding frame FD with a mask in doping the sensing element withimpurities, to prevent the bonding frame FD from being doped with theimpurities.

Further, in the case where the bonding frame FD includes the above-notedlayered structure, it is possible to allow the cap CA to dispense withthe recess portion CAa (in other words, it is possible to employ a flatcap CA for covering the sensing element) by appropriately controlling aheight of the layered structure (please refer to FIG. 5). This wouldeliminate a need of carrying out sandblasting or the like on the cap CAfor forming the recess portion CAa, resulting in reduction inmanufacturing cost.

Second Preferred Embodiment

When anodic bonding is carried out on the acceleration sensorillustrated in FIG. 3, to bond the bonding frame FD and the cap CA toeach other, for example, the mass body MS which is a movable portion ofthe sensing element moves toward a top surface of the cap CA under theinfluence of an electrostatic force in some cases. Such movement maycause the mass body MS to come into contact with the recess portion CAaof the cap CA, which results in anodic bonding also between the massbody MS and the cap CA so that the mass body MS is sticking to the capCA.

One solution to prevent anodic bonding between the mass body MS and thecap CA is to form a metal film 10 in a bottom portion of the recessportion CAa of the cap CA (in other words, on a surface of the cap CAwhich faces the sensing element) as illustrated in FIG. 6. As a resultof the formation of the metal film 10 between the mass body MS and thecap CA, even if the mass body MS comes into contact with the bottomportion of the recess portion CAa of the cap CA due to application of avoltage across the semiconductor substrate SB and the cap CA in anodicbonding between the bonding frame FD and the cap CA, the mass body MSreturns to the original position after the anodic bonding. In thismanner, it is possible to prevent the mass body MS from being stickingto the cap CA.

Nonetheless, the structure illustrated in FIG. 6 is still incapable ofpreventing the mass body MS from moving toward the cap CA under theinfluence of an electrostatic force caused during anodic bonding.Further, an electrostatic force is caused also between the mass body MSand the other elements than the cap CA (the fixed electrodes FE1 andFE2, the shield electrode SE, and the like) during anodic bonding, sothat the mass body MS moves toward, and comes into contact with, theother elements. Such unnecessary movement of the mass body MS duringmanufacture is undesirable

In view of this, in an acceleration sensor according to a secondpreferred embodiment, the metal film 10 is formed in the bottom portionof the recess portion CAa of the cap CA, and further, a portion of themetal film 10 is interposed between the bonding frame FD and the cap CA.In other words, the metal film 10 illustrated in FIG. 6 is lengthened toreach and be connected to the bonding frame FD, as illustrated in FIG.7.

Moreover, the acceleration sensor according to the second preferredembodiment which is illustrated in a plan view of FIG. 8 is differentfrom the acceleration sensor illustrated in FIG. 2 also in that pads PD1e and PD1 f are additionally provided in the surface portion of thesemiconductor substrate SB. Furthermore, interconnect lines LNe and LNfconnected to the pads PD1 e and PD1 f, respectively, are furtherprovided in the surface portion of the semiconductor substrate SB. Theinterconnect line LNe is connected also to the semiconductor substrateSB. Accordingly, the pad PD1 e and the semiconductor substrate SB areconnected with each other via the interconnect line LNe. On the otherhand, the interconnect line LNf is connected also to the bonding frameFD. Accordingly, the pad PD1 f and the bonding frame FD are connectedwith each other via the interconnect line LNf.

It is noted that FIG. 8 illustrates a structure inside the cap CA whenit is seen through the cap CA which is denoted by hidden lines (brokenlines).

In manufacturing the acceleration sensor with the foregoing structure, ametal film 11 electrically connected to all the pads PD1 a, PD1 b, PD1c, PD1 d, PD1 e and PD1 f is formed to extend over all the pads PD1 a,PD1 b, PD1 c, PD1 d, PD1 e and PD1 f, prior to anodic bonding of the capCA to the bonding frame FD. The state where the metal film 11 is formedis illustrated in FIG. 9.

With the metal film 11 being formed as described above, a voltage isapplied between the semiconductor substrate SB and the cap CA toaccomplish anodic bonding.

Even with a voltage being applied between the semiconductor substrate SBand the cap CA during anodic bonding, the semiconductor substrate SB,the metal film 10, the mass body MS, the fixed electrodes FE1 and FE2,the shield electrode SE, the bonding frame FD and the like are held atthe same potential because all of those elements are electricallyconnected via the metal film 11.

Accordingly, no electrostatic force is caused between the elements whichare held at the same potential during anodic bonding. Hence, it ispossible to prevent the mass body MS from being attracted to, and movingtoward, the other elements during anodic bonding. This provides forincrease in yield of an acceleration sensor which is likely to bereduced due to movement of the mass body MS during anodic bonding.

After anodic bonding is finished, the metal film 11 is removed. Theacceleration sensor provided after anodic bonding includes the pads PD1a, PD1 b, PD1 c, PD1 d, PD1 e and PD1 f which are electrically separatedfrom one another, and the metal film 10 connected to a portion of thebonding frame.

Thus, after anodic bonding is finished, each of the pads PD1 a, PD1 b,PD1 c, PD1 d, PD1 e and PD1 f can be used as a pad for wire bonding. Onthe other hand, the metal film 10 is left un-removed after anodicbonding because the metal film 10 can function to block external noisesduring operation of the acceleration sensor. The inclusion of the metalfilm 10 makes it possible to prevent electrical characteristics of thesensing element from being adversely affected by external noises duringoperation of the acceleration sensor.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. An acceleration sensor comprising: a semiconductor substrate; asensing element formed on said substrate; a polysilicon bonding frameformed directly on and in contact with said substrate, wherein saidpolysilicon bonding frame is a monolayer frame, is undoped, andsurrounds said sensing element in plan view; and a cap which includes anend face bonded to a top surface of said polysilicon bonding frame, tocover said sensing element above said sensing element while being spacedby a predetermined distance from said sensing element.